Recently, production of many vegetables and fruits such as fresh vegetable, fruits, flowers and the like has been shifted from open-field culture to planned multiple and diversified culture by using horticultural facilities. Therefore, there are many cases that a large amount of vegetables and fruits must be harvested and packaged within a certain fixed period. Further, even in the case of vegetables and fruits predominantly produced by open-field culture, it is necessary to package and ship a large amount of harvested products within a short period because they have fixed harvest season. In this regard, a major problem during distribution of packaged products to consumers is how to keep vegetables and fruits fresh as just after they have been harvested.
In order to solve the problem, improvement of packaging films and bags for keeping packaged products fresh has been studied intensively and various packaging materials has been proposed.
By the way, the above-described vegetables and fruits do not lose their vital physiological activities immediately after harvesting, but they maintain almost the same activities as just after they have been harvested for a while. And, when they are kept in a proper storage state, their physiological activities can be maintained for a much longer period. As far as such physiological activities are maintained, they are kept in very fresh. In other words, when storage conditions are improper, their physiological activities can not be maintained and they are rotten in a early stage of storage.
As physiological activities of vegetables and fruits in a packaged state, the following phenomena are known:
(1) decrease in water due to transpiration;
(2) consumption of atmospheric oxygen and production of carbon dioxide due to respiration; and
(3) rise in temperature due to production of carbon dioxide and heat generation. Therefore, in a closed state, oxygen content is decreased with increasing in carbon dioxide content, and the inner temperature of a package rises because heat dispersion is inhibited. And, since respiration becomes more active at a certain temperature or higher, the packaged products are liable to rot due to a stuffy atmosphere. In addition, since transpiration becomes more active in an atmosphere at an increased temperature or a lower humidity, the packages products are further liable to rot in the resulting high temperature and high humidity atmosphere.
Then, in order to improve fresh keeping effect upon storage from the viewpoint of the above-described physiological activities, it has been attempted to improve gas permeability of a packaging bag having low gas permeability, for example, by providing perforation of an appropriate size to the bag or by cutting the bottom of the bag to prevent decrease in oxygen content and increase in carbon dioxide content as well as rise in temperature. However, such an attempt is a mere "first aid", and the distribution of temperature and the gas composition in a package are not always uniform. Therefore, rot progresses locally, particularly, at an inner portion which can not be observed from the outside. This may unexpectedly commit a fraud upon consumers. By the way, depending upon a particular kind of vegetables and fruits, perforation is not always preferred because a packaged product may be invaded with vermin and/or be wetted through perforation to deteriorate quality.
Under these circumstances, it has been requested to develop a packaging film which can maintain the inner temperature and the gas composition of a package suitable for physiological activities of particular vegetable and fruits to be packaged. However, such a packaging film, has not been proposed heretofore in the prior art. Further, packaging bags for vegetables and fruits produced by using presently available packaging films cause fogging due to condensation of moisture which is derived from the above-described transpiration or evaporation of free moisture on the inner surface of the bags to adversely affect appearance of packed products.
For, example, in the case of a polyethylene film, although it has a desired water vapour transmission, its oxygen and carbon dioxide permeabilities are insufficient and therefore vegetables and fruits are stifled to become less fresh within a short period of storage. In the case of a polystyrene film, although it has a desired oxygen and carbon dioxide permeabilities and can maintain respiration of vegetables and fruits for a long period of time, its water vapour transmission becomes too great, which results in activation of moisture transpiration to cause discoloration or wilting of vegetables and fruits within a short prior of time therefore it is difficult to keep them fresh for a long time.
In addition, the above-described known films are deficient in anti-fogging properties, which results in less commercial value due to bad appearance and, further, so-called wet rot caused by contact of vegetables and fruits with condensed water at a fogging part.